Polyamide resins are recently attracting the attention of researchers as materials for producing more durable and lighter automotive hoses such as gasoline supply hoses. However, since polyamide resin hoses are not as flexible as rubber hoses, efforts are being made to provide high flexibility to the polyamide resins by one of the following three methods: the addition of a plasticizer to the polyamide resin; blending rubber with the polyamide resin; and block and/or graft polymerizing a dissimilar monomer onto the polyamide resin. However, the use of a plasticizer is not practically feasible because the plasticizer vaporizes with heat or is extracted with gasoline and other oils or solvents, causing a significant drop in the flexibility of the polyamide resin. The block and/or graft copolymer prepared by the third method has a low melting point and is not suitable for high-temperature use which is the principal intended application of the polyamide resin.
In order to permit use in applications where contact with gasoline and other oils or solvents is expected, the rubber for use as component to be blended with the polyamide resin in the second method is selected from acrylonitrile-butadiene copolymer rubber (for convenience this will simply be referred to as nitrile rubber) and epichlorohydrin rubber (hereunder simply referred to as hydrin rubber). Japanese patent Publication No. 14096/1980 and U.S. Pat. No. 4,173,556 show a method in which a blend of the nitrile rubber and polyamide resin is mixed with a cross-linking agent with heating so that the nitrile rubber is dispersed in the polyamide resin while the rubber is being cross-linked. Similarly, Japanese Patent Application (OPI) No. 5753/1982 and U.S. Pat. No. 4,297,453 show a method in which a blend of the hydrin rubber and polyamide resin is mixed with a cross-linking agent with heating so that the hydrin rubber is dispersed in the polyamide resin while the rubber is being cross-linked. However, the nitrile rubber impairs significantly an inherent high resistance to thermal aging of the polyamide resin, further the hydrin rubber reduces the good strength properites of the polyamide resin.
The present inventors previously proposed in Japanese Patent Application No. 41576/1983 (corresponding to U.S. application Ser. No. 589,223 filed on Mar. 13, 1984) a method for preventing the drop in the resistance to thermal aging and strength properties of a polyamide resin by blending a polyamide resin with two other polymers, i.e., nitrile rubber and hydrin rubber. However, the resulting blend is inferior to the polyamide resin per se in respect of resistance to ozone, upon the cyclic application of large strains, and resistance to stress whitening under appreciable local strains. Therefore, this three-polymer blend is not satisfactory for use for parts which are subject to significant vibrations or extreme strains.
Therefore, the present inventors continued their studies on producing a polyamide-rubber blended composition which has improved resistance to ozone and stress whitening, as well as high strength and flexibility, and significant resistance to thermal aging as well as gasoline. As a result, it has been found that the desired composition can be produced by the present invention.